In numerous applications, thin coatings of alloy materials are used on metal structures to protect them from the detrimental effects of high temperatures and/or corrosive environments. For example, in a gas turbine engine the turbine buckets (i.e., the turbine blades) and other components must be able to withstand exposure to extreme high temperature-high velocity corrosive gas streams. A protective coating on all components exposed to the gas stream is necessary to insure the longevity and satisfactory performance of the turbine. In many conventional turbines, the protective coating is developed from a class of alloys known as MCrAlY coatings, wherein M is selected from the group of metals consisting of cobalt, iron, nickel and mixtures thereof. Such coating compositions are well known in the art, as described, for example, in U.S. Pat. No. 4,419,416. Vacuum plasma sprayed coatings employing platinum-aluminum compositions (e.g., LDC-2E) are also known and used.
Some of the problems inherent in the manufacturing and refinishing of components having protective metal alloy coatings are discussed by the present inventor in U.S. Pat. No. 4,920,319. For example, in manufacturing new components having protective metallic or semiconductive material coatings and for deciding whether to rework such existing coatings when servicing used components, it is desirable to be able to determine both the thickness of the coating over the surface of the component and the presence of any cracks in the coating. Specifically, coatings on gas turbine buckets and other metal components are subject to thickness variations during manufacture and to the development of craze cracks during service which may penetrate into the bucket substrate material itself. It follows that in both the original manufacture and in refurbishing of turbine buckets, it is important to know the thickness of the bucket coating at all surface points as well as the location and depth of any existing coating cracks in order to determine whether the cracks penetrate the coating into the underlying bucket substrate material. Knowing such information, one can make informed decisions regarding the quality of the manufacturing process or the need for bucket repair. For example, if a crack penetrates the surface only to the extent of the coating thickness, the flaw can be remedied by a chemical stripping process that removes the coating and prepares the part for re-coating.
For obvious practical and economic reasons, it is most desirable to obtain such information using nondestructive testing methods. However, conventional nondestructive inspection methods are not capable of such comprehensive evaluations of coated components. For example, a fluorescent penetrant is used in one conventional method for nondestructive testing of such parts. However, using this method, parts with coating craze cracks typically show innumerable fluorescent crack indications rendering a comprehensive analysis impractical. Moreover, this conventional method is incapable of determining either crack depth or coating penetration.
Although eddy current testing is capable of relating crack depth information, when used as a stand-alone inspection tool and without the benefit of coating thickness information, it cannot determine whether or not a crack has penetrated the coating into the underlying substrate. Similarly, conventional thermoelectric testing methods provide only coating thickness information. As such, comprehensive information concerning characteristics of the coating and any cracks in it cannot be readily determined from a single methodology.
It is therefore an object of the invention to provide a computerized thermoelectric and eddy current combined test system for nondestructive testing of metallic or semiconductive material coated (or coated and diffused) components. It is a further object of the invention to provide a computerized system for the collection, analysis, evaluation and display of material surface coating thickness and surface flaw data for turbine or other machine components. It is also a further object of the invention to provide an improved eddy current probe design for use in such testing systems to obtain accurate eddy current measurements along a leading or trailing edge of a turbine bucket.
In accordance with a preferred exemplary embodiment of the present invention, a method and apparatus is provided for the acquisition and the correlation of eddy current test data with other forms of measured test data so that a more comprehensive analysis of the condition of a gas turbine bucket is possible. In particular, eddy current data obtained from a surface scan of a turbine bucket is combined and correlated with thermoelectric scan data of the same bucket surface. Surface scanning on separate occasions by eddy current and thermoelectric probes is computer controlled to obtain an exact correspondence between recorded data and the surface position of each probe during each scan. In addition to providing scan control and coordinate information for the probes, the testing system computer controls data collection, data storage (e.g., on a hard disk or an optical disk for archival purposes), data reduction, and production or display of data in various selectable forms such as charts, graphs, color-coded surface maps or printed hard copies.